Removal of contaminants from semiconductor wafers is a matter of great concern in integrated circuit manufacturing. As the critical dimensions of circuit features become ever smaller, the presence of even a minute foreign particle on the wafer during processing can cause a fatal defect in the circuit. Similar concerns affect other elements used in the manufacturing process, such as lithographic masks.
Various methods are known in the art for stripping and cleaning foreign matter from the surfaces of wafers and masks, while avoiding damage to the surface itself. For example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated herein by reference, describes a method and apparatus for removal of particles from solid-state surfaces by laser bombardment. U.S. Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also incorporated herein by reference, describe a method and apparatus for removing surface contaminants from a substrate by high-energy irradiation. The substrate is irradiated by a laser with sufficient energy to release the particles while an inert gas flows across the wafer surface to carry away the released particles.
U.S. Pat. No. 4,987,286, whose disclosure is likewise incorporated herein by reference, describes a method and apparatus for removing minute particles (as small as submicron) from a surface to which they are adhered. An energy transfer medium, typically a fluid, is interposed between each particle to be removed and the surface. The medium is irradiated with laser energy and absorbs sufficient energy to cause explosive evaporation, thereby dislodging the particles.
One particularly bothersome type of contamination that is found on semiconductor wafers is residues of photoresist left over from a preceding photolithography step. U.S. Pat. No. 5,114,834, whose disclosure is incorporated herein by reference, describes a process and system for stripping this photoresist using a high-intensity pulsed laser. The laser beam is swept over the entire wafer surface so as to ablate the photoresist. The laser process can also be effected in a reactive atmosphere, using gases such as oxygen, ozone, oxygen compounds, nitrogen trifluoride (NF3), etc., to aid in the decomposition and removal of the photoresist.
European patent publication EP 0 943 390 A2, whose disclosure is incorporated herein by reference, describes a method of surface treatment using multi-laser combustion, providing an improvement on the method of the above-mentioned U.S. Pat. No. 5,114,834. The mechanism of photoresist removal is described as including a localized ejection of the photoresist layer, associated with a blast wave due to chemical bonds breaking in the photoresist and instant heating of the ambient gas. A fast combustion flash of the ablation products occurs due to the photochemical reaction of ultraviolet (UV) laser radiation and the process gases, which may also be due to the blast wave. The combination of laser radiation and fast combustion provides instantaneous lowering of the ablation threshold of hard parts of the photoresist.
Another method for photoresist removal is described in an article by Y. P. Lee, et al., entitled “Steam Laser Cleaning of Plasma-Etch-Induced Polymers from Via Holes,” in the Japanese Journal of Applied Physics 37 (1998), pp. 3524-3529, which is incorporated herein by reference. An alcohol film is applied to the insides of the via holes prior to irradiation by an excimer laser. Explosive evaporation of the alcohol caused by the laser pulse was found to improve the efficiency of removal of the photoresist from the holes.
PCT patent publication WO 96/06694, whose disclosure is incorporated herein by reference, describes a method for performing surface processing of flat panel display substrates, and particularly for removing photoresist after a photolithography step. A high-intensity pulsed beam of radiation is swept across the surface while a reactive gas is flowed across it. The radiation causes the photoresist material to react with the gas, resulting in gaseous products that are then drawn away from the surface. U.S. Pat. Nos. 5,669,979 and 5,814,156, whose disclosures are likewise incorporated herein by reference, describe further methods of cleaning a substrate surface photoreactively, in a manner that is purported to avoid damaging the surface. A laser beam of UV radiation is swept across the surface, while a flow of a reactant gas is provided in a reaction region so that the gas is excited by the UV laser beam.
U.S. Pat. No. 5,023,424, whose disclosure is incorporated herein by reference, describes a method and apparatus using laser-induced shock waves to dislodge particles from a wafer surface. A particle detector is used to locate the positions of particles on the wafer surface. A laser beam is then focused at a point above the wafer surface near the position of each of the particles, causing an electrical breakdown in the gas above the surface that produces gas-borne shock waves with peak pressure gradients sufficient to dislodge and remove the particles.